On Wed, 05 Mar 97 00:08:00 PST Cecil A Moore
<Cecil_A_Moore@ccm.ch.intel.com> writes:
>>From: dwink@juno.com (Daniel C Winkler)
>>Any integer-multiple of half-wavelengths of coax feedline will appear
>>to be a HIGH impedance, and it WILL NOT radiate. A half-wave-length
>>coax feedline will not radiate. Assuming you have it attached to
>>the middle of a resonant dipole.
>
>Hi Daniel, I do believe I smell a myth.
Hey, let's not get personal! I just took a bath!
> We are talking about the
>outside of the braid which looks just like a piece of wire to the
>emerging RF from the inside of the braid. A half-wavelength of wire
>repeats the voltage, current, and impedance at both ends
Yup, that's right, so the end of this wire (the coax) is gonna look
just like our feedpoint, and it's gonna have a few volts of rf on it
(like maybe 25 peak, for a full 5 watt output and something close to a
resonant dipole). So what?? The presence of the rig and operator
will detune that half-wavelength a little, of course, but so what? The
operator will be hot, but the line won't radiate. 25 volts of rf won't
hurt cha.
> and one end is grounded
Grounded? Grounded???? This is a *camping* antenna. He's
gonna lug an 8' copper-clad GROUND rod up the switchbacks? I think
not! The radio is just going to be a little hot, that's all. The
chassis will be at a few volts.. of..
.. rf...
potential... . . .
(Inside Dan's head a dim lightbulb begins to glow...)
(Opps...where is that rf coming from?)...
(From the rig, dummy!)...
(Where is the rf potential coming out of the rig??)...
(Why, it's coming out of the rf connector and going up t h e c o a x
a n d . . . ). . .
(...Yes, and?) . . .
(And I have a funny taste in my mouth!)...
(Shoe leather, perhaps?)
Aagggg! Foot-in-mouth disease! Pteuw. Pthew. Cecil is RIGHT, my friends. I stopped thinking about a half second too soon the
other night. A half-wave WIRE will not radiate. It will indeed look
like a high impedance and will accept no appreciable rf if attached to
the feedpoint of an antenna. But this half-wavelength of coax is NOT a simple half-wavelength of wire, because it IS at RF-ground at its
far end (the rig end)- BY DEFINITION.
Does everybody understand this? Please, if you are confused by
this interchange, please post to the list, and Cecil or I (or others)
will try to explain it again. It is a pretty fundamental idea.
(First try re-reading the above, now that you are not laughing so hard
at my beet-red face.)
And did you notice that I immediately assumed that Cecil meant
EARTH ground? That's because a great many hams incorrectly think a great chunk of metal sunk in the earth has something to do with getting
their rf out. Not so; earth grounds are for lightening protection.
And I should KNOW that Cecil knows that, but I had convinced myself so thoroughly that the half-wavelength of coax wouldn't radiate, that I
temporarily thought Cecil was also falling prey to that old confusion
between electrical (or rf) ground and earth ground. Dumb, dumb. dumb.
Once again I learn that I should not post to the list past my
bedtime, and that I should review all my posts before sending them the next day, because just maybe I've stepped in *it* just one more time!!
(Although it was such FUN to have caught Cecil in a little goof, I
probably would have deluded myself on a re-read as well.)
Thank you, Cecil, for being gentle. I hope someone benefits
from all this egg on my face!
PS, if I'm not mistaken, there is a corollary to the above. A
QUARTER wavelength coax feedline should not radiate at all,
theoretically, since its far end (at the rig) is forced to be at rf
ground potential. That would be a whole lot more convenient, anyway.
On 40m., 66' of feedline was too long, but 33' is pretty good for a
field antenna. I still think you need a choke balun, though, just as
Cecil suggested. After all, the rig is not a point-source rf
generator, so having your feedline a quarter wavelength long might
greatly exaggerate any little rf ground loops inside your rig and lead
to some instabilities that wouldn't otherwise appear.
73, ; D DWink@Juno.com Dan Winkler N7IVR Seattle, WA
----------whom the gods would destroy, they first make proud ----------
--------------------------ZAP!----------------------------
[
QRP-L Archive |
]
[
1993 |
1994 |
1995 |
1996 |
1997 |
1998 |
1999 |
2000
]